exposure impact on ISO variation

maybe not such a trivial question, but I am unsure about the following:

it is common knowledge especially in underwater shooting (all manual, no such thing like TTL) that the flash exposure at a given flash strengths is controlled by the aperture and thus affecting the foreground, while the ambient light (background, environment) is controlled by the shutter speed.

If I have a setting to start e.g. f8 and 1/125 sec at ISO 100 and by this my foreground is well exposed but the background is too dark, then I could go to 1/60 sec. If for some reasons I do not want to change the shutter speed but want to increase the ISO from 100 to 200 would that change my ambient exposure of the background AND also my flash exposure of the foreground? Or only the background by 1 full stop? or both by half a stop?

It is clear to me that exposing without flash a f8 1/125s at ISO 100 configuration would yield the same as f11 1/125s at ISO 200 or f8 1/250s at ISO 200 (that is a 1 stop difference each). This is not regarding DOF by the changing aperture or freezing effects by the changing shutter speed, just for the exposure value.

But how is it with the flash?

other example:

let's assume I have a good exposure for background and foreground with settings like f8 1/125s at ISO 200 with a half power of the flash. What is the aperture and shutter speed if ISO is changed to 100 (same flash power) which gives me the same overall exposure than in the beginning?

thanks for enlightenment!

As you increase your ISO setting you can decrease the flash output by a corresponding amount and keep the same f-stop. The challenge is that the clicks on many strobes may not correspond to full stops, rather something fractional. You will have to determine by trial and error how many clicks you need to adjust on your particular strobe. You may find that doubling the ISO does not work too well and that an intermediate setting works better. For example increasing your ISO from 100 to 160 corresponds to a one click decrease in your strobe output. You will have to find out by experimentation. This is much easier with digital since one can chimp and not wait for film development.

and what about the shutter speed for the ambient? did you forget about that?

Also by 1 stop if changing the ISO 1 stop? or half a stop in aperture plus half a stop in shutter when ISO is changing by 1 stop?

BTW, some flash lights do pretty well with 1/8, 1/4, 1/2 and full power e.g. Subtronics.

thanks

 

 

and what about the shutter speed for the ambient? did you forget about that?

 

Also by 1 stop if changing the ISO 1 stop? or half a stop in aperture plus half a stop in shutter when ISO is changing by 1 stop?

BTW, some flash lights do pretty well with 1/8, 1/4, 1/2 and full power e.g. Subtronics.

thanks

If the background is too dark like in your above example and increasing ISO fixed it, then keep the shutter speed the same. You can also make the shutter speed longer and keep the ISO the same. There are many solutions. Yes, having more clicks (settings) on your strobe increases the number possibilities such as smaller changes - all trial and error.

I tend to stick with one setting rather than mess with all of the above. I do not like to use full power since that slows down the recycling rate so one click down is the max I will use - which is 1/2 power on my Seacam 150D, I adjust the ISO to work with that setting and use as fast as shutter speed as possible. For example in 2013 I shot thousands of pix with my old D2X and 10.5mm lens with the 150D strobe all with the same shutter speed, ISO, and aperture. I used the shutter speed and aperture lock feature on the camera to avoid accidentally changing it. In fact it may still be locked as I am not using the body for anything other the the 10.5 and mini dome.

I understand it now in the following way:

either change the aperture (by 1 stop in the example) and leaving shutter speed and flash output unchanged

or

leave aperture unchanged but change flash power output and shutter speed both by 1 stop

Why do I ask this?

I have had an interesting discussion recently with Kurt Amsler (a well known underwater photographer) and he told me about today's new concepts regarding the virtual no noise sensor cameras at higher ISOs like ISO 400 (I have a D4s now).

Think about the following scenario:

You shoot a dynamic scene (let's say fast moving sharks) in relative deep water with dim ambient lighting conditions. You shoot at f8 and 1/125 sec, 2 flash guns at half power each. The image on the LCD looks ok on the exposure of the shark (foreground) but is too dark on the background, almost black and thus rather underexposed (let's say by -1 stop). But we want blueish water !

Now, instead of lengthening the shutter speed to 1/60sec you can do as alternative: change ISO from 100 to 400 (2 stops) plus lowering the flash output to 1/8 power and thus reducing backscatter and decreasing flash recycle time, leaving the aperture unchanged (relative closed at f8) thus conserving DOF (nobody shoots today with f2.8 - 4.0 anymore!), shorten the shutter speed to 1/250sec and thus freezing the action of the moving subjects even more...

Off course, that works only with a camera which can handle ISO 400 on a neglectable noise level.

Wonderful world, isn't it ?

Anybody else to chime in?

High ISO is certainly a way to go. I have been using high ISO even with older cameras like the D2X as it is relatively dark here in Alaska even in mid-summer when under water. I just have to accept more noise. You might also consider using Magic filters http://www.magic-filters.com/ with high ISO and no flash, should work even better with the newer high ISO capable cameras.

ISO changes apply equally to the entire frame foreground and background

I have experiences of shooting sharks in the galapagos as this one

Galapagos Shark School by Interceptor121, on Flickr

What I did was as follows:

1. Set aperture based on lens and subject distance in this case f/4 (the sharks were 5 feet away and I shoot a compact with flat wide lens in this case)

2. Set shutter speed based on 1/60 as they were cruising slowly

3. Then chose an ISO that would give me the blue I wanted in this case 200

4. Set the strobe at half power to avoid highlights on the shark belly

Backscatter (other than really close to the strobe) is only determined by arm length, strobe beam angle and subject distance

If I had been using an SLR with a rectilinear zoom lens for this shot I would have required at least an f/8 to have decent sharpness at the corners which would have meant ISO 800